Assuming that you're talking about the common "tree of heaven," ailanthus altissima, all over North America it's considered one of the most invasive and pernicious of weed trees. In other words, almost nothing bothers it short of heavy doses of herbicide, cutting down, poisoning of roots, and digging out of stumps. If you're talking about some other kind of tree, an established plant that suddenly starts to lose leaves is likely being affected by some change in environment (water and light being the two major ones,) or by pests or pathogens. You could get samples of the leaves and soil analyzed through your local Master Gardener program. This link will help you locate one: http://mastergardeners.org.uk/get-involved/find-your-nearest-master-gardener/